Bottled water is everywhere — in backpacks, on desks, in cars. When I was younger, no one carried water unless they were going hiking.

Why the demand for bottled water now? I have a theory.

Americans eat about twice the salt they did three to four decades ago. Most of it is hidden in all the canned, boxed, packaged and processed foods we open at home and eat in restaurants.

Eating excess salt makes you thirsty and causes your body to hold more water. It’s like filling up a water balloon. As your body holds more water, your blood pressure goes up. Your heart has to work harder to push all that fluid around, increasing your risk for a heart attack or stroke. This is especially likely if your kidneys and heart are not at tiptop shape.

The solution, of course, is not to stop drinking water; it is to stop eating salt. People who eat the recommended limit of 1,500 milligrams to 2,000 milligrams of sodium per day are less thirsty. They drink an average of 3 to 4 cups of fluid a day, and experience no thirst.

Double your salt intake, to the average U.S. intake of about 4,000 milligrams a day, and your fluid needs to zoom to 8 to 10 cups a day. With processed food it happens easily. You can blow 1,000 milligrams of sodium on just one cup of packaged mac-and-cheese, one cup of canned soup or a Big Mac. Soon you have to start lugging around a water bottle.

I also have trouble with the idea of buying bottled water.

First off, our tap water is safe. We have some of the purest water in the country. And it’s a bargain. I pay about a third of a penny per gallon of water from my tap. Bottled water is $5.50 a gallon — more than gas for my car.

Tap water must meet Environmental Protection Agency standards. But bottled water is overseen by the U.S. Food and Drug Administration and labeled like a food. So you get information such as “zero calories,” but no word about whether the water is clean and free of bacteria or trace minerals.

Water bottles make up about 1.5 million tons of plastic waste each year, 80 percent of which goes to the landfill. Plastic comes from petroleum, a scarce resource. Bottle-making uses enough oil to fuel about 100,000 cars for a year. Getting bottled water to thirsty drinkers costs another 200,000 car/years of fuel.

In many cases, the energy required to move the bottle from the source to my home is more than it took to make the bottle in the first place.

How can you help the planet while improving your health? It all comes down to salt.

Cut back the salt in your diet to 1,500 milligrams to 2,000 milligrams each day. At a healthy salt level, you naturally won’t be thirsty. You will drink less and lose some of that “water balloon” you have been carrying. Your heart and kidneys will thank you. And so will the planet.

Katy G. Wilkens is a registered dietitian and head of the nutrition department at Northwest Kidney Centers. See her recipes at www.nwkidney.org.